This invention relates to a fluid power system.
In their most basic form, fluid power systems generally consist of a pressurised fluid source, a motion control valve and an actuator such as a ram or a motor. Systems are typically generalised by attaching further motion control valves in parallel to the first so that additional actuators can be moved with the power supplied by the fluid source. Because most actuators have a fixed linear or rotary movement per unit of fluid displacement, the force they exert is directly proportional to the pressure supplied. In systems with a single pump and multiple actuators there is always undesirable compromise given the practical impossibility of matching the instantaneous pressure requirements of all of the active actuators to the single pressure supply.
In the case of the state-of-the-art “load sensing” system, the displacement of a variable displacement pump is controlled such as to maintain its output pressure to a fixed margin above the maximum pressure required of any of the loads. The difference between this pressure and the actual pressure required of any one of the loads is throttled in a proportional valve, creating energy losses. When only one actuator is moved at a time these systems can be reasonably efficient. However when multiple actuators must be moved simultaneously at different pressures then the efficiency becomes poor—depending on the duty cycle, these losses can cause the overall efficiency of such a system to reduce to 30%.
The pump/motor described in EP 0494236 B1 and sold under the trade mark Digital Displacement is a positive-displacement fluid pump/motor in which the working volumes are commutated not by mechanical means but by electronically-controlled solenoid-actuated poppet valves. Control of flow is achieved by varying the time-averaged proportion of working volumes which are commutated such as to pump fluid from the low pressure port to the high pressure port (“pump enabled”), or which are commutated such as to motor fluid from the high pressure port to the low pressure port (“motor enabled”), to the proportion which are connected in both expansion and contraction strokes to the low pressure port and thus do no fluid work (“idled”). A controller, synchronised to the position of the shaft by means of a position sensor, supplies pulses to the solenoid coils at the appropriate times such as to commutate each working volume as desired. Because the commutation of each stroke of the working volume is independently controllable, the pump/motor is capable of supplying fluid to or absorbing fluid from a port, in individual discrete volume units, each corresponding to a single stroke or part of a stroke (see WO 2004/025122) of a single working volume. The high pressure port of each working volume may be connected to a different fluid circuit. Thus a single pump/motor composed of many working volumes may provide multiple independent fluid supplies or sinks, the flow to or from each of which is independently variable.
WO 2006/011836 describes a system in which two separate pumps can be connected to first and second load outlet points in different configurations.